The present invention relates to a color image reading apparatus that efficiently processes analog signals during image reading; more specifically, the apparatus that can read monochrome image at a high speed.
A major principle of image reading in most recent color image reading apparatus is reading an image with a Charge Coupled Device (hereinafter referred to as CCD) line sensor splitting into the R(red), G(green) and B(blue) color components, and scanning the CCD line sensor in the direction of subsidiary scanning. For increasing the processing speed, the CCD line sensor delivers signals of the respective R, G and B pixels separated into the signal the even-number pixels and that from the odd-number pixels.
The structure and the operation of a conventional image reading apparatus is described in the following. FIG. 17 illustrates a CCD line sensor in a conventional image reading apparatus.
As shown in FIG. 17, a CCD line sensor 6 delivers analog output signals of pixel rows 101a, 101b and 101c corresponding respectively to R, G and B, each separated into the signal from the even-number pixels and that from the odd-number pixels.
FIG. 18 illustrates outline structure of a conventional image reading apparatus, with respect to the optical system and the mechanical section.
The portion of optical system and mechanical section comprises a transparent glass platen 1 for having an original document on the top surface, a lamp 2 for illuminating the document, a mirror 4 for guiding the light reflected from the document to lens 5, a lens 5 for focusing the light reflected by the mirror 4, a CCD line sensor 6 for reading the focused light in terms of electric charges through photoelectric conversion, and a carriage 3 that transfers the CCD line sensor 6, together with the lamp 2, the mirror 4 and the lens 5, in the direction of subsidiary scanning for scanning the lines of document.
FIG. 19 shows a block diagram of an analog data processing sector of a conventional image reading apparatus.
An analog data processing sector of the image reading apparatus comprises, as shown in FIG. 19, a gain corrector 7 for correcting the amplitude of the analog signal outputted from CCD line sensor 6, an offset corrector 8 for correcting the DC potential of the analog output signal, a sample holder 9, a multiplexer 10a, 10b, 10c for time sharing and multiplexing the analog signal, an A/D (analog-digital) converter 11a-11c for converting the analog signal to digital signal, a dropout color selector 12 for selecting a data among the input data in accordance with control signal and outputting the selected data, a timing generator 13 which outputs the timing signal for driving the CCD line sensor 6, the sample holder 9, the multiplexer 10a-10c and the A/D converter 11a-11c in accordance with the output of control signal, and a reading mode controller 14 which delivers control signal for controlling the dropout color selector 12 and the timing generator 13 in accordance with instructions inputted through a CPU (not shown) or an operation panel (not shown) for switching a reading mode, viz. reading in color or in monochrome, and specifying a dropout color for the reading in monochrome.
Descriptions will follow referring to a conventional image reading apparatus shown in FIGS. 18-20. FIG. 20 shows a timing chart at the reading in color mode and the reading in monochrome mode. FIG. 21A shows structure around the multiplexer section in a conventional image reading apparatus. FIG. 21B shows operational logic of the multiplexer of a conventional image reading apparatus.
A document placed on the surface of glass platen 1 is illuminated with the lamp 2. The light reflected from the document is guided by the mirror 4 to the lens 5, and the light focused by the lens 5 is split by R, G and B color filters (not shown) into the R, G and B color components. The respective color components are read by the CCD line sensor 6.
The CCD line sensor 6 is driven by output of the timing generator 13, which generates a certain waveform in accordance with instruction from the reading mode controller 14. Analog signals outputted from the CCD line sensor 6 undergo a correction, in the amplitude and the DC potential, at the gain corrector 7 and the offset corrector 8 to assume a waveform that meets the input width of the A/D converter 11a, 11b, 11c. By the sample hold timing of FIG. 20, only the effective output is held in the sample holder 9.
Analog signals of respective R, G and B outputted from the sample holder 9 are time shared and multiplexed at the multiplexer 10a-10c into the signal of the even-number pixels and that of the odd-number pixels individually, in accordance with the waveform of the multiplexer xe2x80x9canxe2x80x9d, shown in FIG. 20, under the logic of FIG. 21B. Each of the analog signals outputted from the multiplexer 10a-10c is delivered respectively to the A/D converter 11a-11c to be converted into digital signal, which signal is then delivered to the dropout color selector 12.
In the reading in color mode, the dropout color selector 12 outputs the respective R, G and B digital data through as they are. When reading in monochrome mode, the dropout color selector 12 outputs only a color component designated by the reading mode controller 14.
Assume R is designated as the color for the reading in monochrome mode, then only the R data is outputted. Digital signal outputted from the dropout color selector 12 undergoes a certain predetermined process such as the line correction, chrominance correction, etc., before it is delivered to a later stage.
After the CCD line sensor 6 has read data for one line in a predetermined accumulation time, the carriage 3 moves by one line in the subsidiary scanning. Repeating this operation completes scanning of an entire page of the document.
In the above-described method of the image reading in monochrome mode, a data of the dropout color is selected after the A/D conversion. If R is selected as a dropout color, for example, only the R data is outputted after all of the R, G and B data have been A/D converted.
Namely, the A/D converters involved in the G and B conversion are conducting a useless data conversion. The reading in monochrome mode is generally performed at a speed higher than that in color. This means that the speed required for an A/D converter is faster in the reading in monochrome mode than that in the reading in color mode.
In other words, an A/D converter having a high conversion speed satisfying a speed required for the reading in monochrome is compelled to make a slow conversion at the reading in color mode. Thus, the capacity of A/D converter is partly left idle in an image reading apparatus that has both a color reading mode and a high speed monochrome reading mode. This eventually leads to a higher cost of the apparatus.
The present invention addresses the above drawbacks of the conventional image reading apparatus. The present invention aims to offer an image reading apparatus equipped with both a color reading mode and a high speed monochrome reading mode, in which the dropout color is selected at the stage of analog signal in order not to leave the capacity of an A/D converter partially idle. The image reading apparatus of the above structure can read an image at a high speed, yet at a low cost.
An image reading apparatus of the present invention comprises:
selecting means that outputs, for the reading in color mode, the analog signals of respective color components delivered from sample holding means after time sharing and multiplexing the signal of even-number pixels and that of odd-number pixels individually, while for the reading in monochrome mode, analog signal of selected color component, or luminance signal generated from the analog signals of respective color components delivered from the sample holding means, separated in the row of even-number pixels and the row of odd-number pixels;
signal converting means for converting the analog signal multiplexed in the selecting means into digital signal, or converting analog signals of the even-number pixels and the odd-number pixels that have been individually outputted into digital signals respectively; and
rearranging means that outputs, in case of the reading in color mode, the digital signals converted by the signal converting means as they are, while in case of the reading in monochrome mode, rearranges the digital data rows that have been separated into that of even-number pixels and that of odd-number pixels into a single sequential data row.
By the above processing, the speed of an A/D converter may stay low even when the reading in monochrome mode is conducted at a speed higher than that in color mode. For example, if the reading in monochrome mode is conducted twice as fast that in color mode, the A/D conversion may be performed at the same speed as in the reading in color mode.